Function save_cache 

pub fn save_cache(
    workspace: &WorkspaceState,
    tree_selected: usize,
    flat: &[FlatEntry],
    project_touched_unix_ms: &HashMap<PathBuf, u64>,
    stale_collapsed_projects: &HashSet<PathBuf>,
    dismissed_integration_prompts: &HashSet<IntegrationTarget>,
    sync: bool,
) -> Option<String>